For more than 30 years, spinal cord stimulation (SCS) has been used to treat a variety of pain syndromes. The goal of SCS is to create paresthesia that completely and consistently covers the painful areas, yet does not cause uncomfortable sensations in other areas. Paresthesia may be defined as a sensation of tingling, pricking, or numbness in an area of the body. It is more generally known as the feeling of “pins and needles”. In some instances, the feeling of paresthesia is preferred over the feeling of pain. In SCS, paresthesia production is accomplished by stimulating Aβ fibers in the dorsal column and/or the dorsal roots. Dorsal column stimulation typically causes paresthesia in several dermatomes at and below the level of the stimulator. In contrast, dorsal root stimulation activates fibers in a limited number of rootlets in close proximity to the stimulator and causes paresthesia in only a few dermatomes. Because of these factors, dorsal root stimulation with an SCS stimulator may not produce sufficient pain relief. In addition, stimulation of the roots with an SCS stimulator can cause uncomfortable sensations and motor responses. These side effects may occur at pulse amplitudes that are below the value needed for full paresthesia coverage. Therefore, the clinical goal of SCS is to produce an electrical field that stimulates the relevant spinal cord structures without stimulating the nearby nerve root.
Intraspinal nerve root stimulation is a technique related to SCS, except that electrodes are placed along the nerve rootlets in the lateral aspect of the spinal canal (this area is known as “the gutter”), rather than over the midline of the spinal cord. The electrodes are mounted on a cylindrical lead rather than on a traditional SCS paddle lead. The accuracy of the leads' placement within the gutter is confirmed by stimulating the nerve roots at perceptible levels, which result in paresthesia in the local area. Sensory paresthesia may be generated by stimulating at a level above the threshold for sensory recruitment. This may be used in conjunction with SCS to treat certain pain conditions.
For some patients, paresthesia is an undesired effect and is not a well tolerated alternative to pain. Therefore, improved treatments are needed to provide pain relief with minimal undesired effects. At least some of these objectives will be met by the present invention.